| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1961 | 76.0 | 81.0 | 87.0 | 89.0 | 92.0 | 93.0 | 97.0 | 100.0 | 100.0 | 90.0 | 86.0 | 78.0 | 100.0 |
| 1962 | 78.0 | 84.0 | 84.0 | 86.0 | 94.0 | 96.0 | 99.0 | 103.0 | 98.0 | 95.0 | 84.0 | 78.0 | 103.0 |
| 1963 | 78.0 | 79.0 | 87.0 | 94.0 | 96.0 | 101.0 | 103.0 | 103.0 | 101.0 | 96.0 | 85.0 | 74.0 | 103.0 |
| 1964 | 74.0 | 73.0 | 80.0 | 89.0 | 95.0 | 97.0 | 104.0 | 107.0 | 99.0 | 87.0 | 83.0 | 80.0 | 107.0 |
| 1965 | 80.0 | 72.0 | 78.0 | 90.0 | 91.0 | 97.0 | 100.0 | 103.0 | 100.0 | 91.0 | 87.0 | 75.0 | 103.0 |
| 1966 | 74.0 | 72.0 | 84.0 | 88.0 | 92.0 | 95.0 | 102.0 | 98.0 | 95.0 | 92.0 | 81.0 | 80.0 | 102.0 |
| 1967 | 76.0 | 78.0 | 91.0 | 89.0 | 94.0 | 98.0 | 98.0 | 101.0 | 95.0 | 92.0 | 84.0 | 79.0 | 101.0 |
| 1968 | 72.0 | 75.0 | 80.0 | 86.0 | 92.0 | 96.0 | 95.0 | 96.0 | 93.0 | 90.0 | 85.0 | 74.0 | 96.0 |
| 1969 | 80.0 | 78.0 | 81.0 | 86.0 | 94.0 | 98.0 | 102.0 | 105.0 | 98.0 | 94.0 | 86.0 | 75.0 | 105.0 |
| 1970 | 80.0 | 79.0 | 78.0 | 88.0 | 90.0 | 99.0 | 101.0 | 103.0 | 97.0 | 91.0 | 80.0 | 80.0 | 103.0 |
| 1971 | 84.0 | 78.0 | 88.0 | 87.0 | 91.0 | 99.0 | 103.0 | 98.0 | 98.0 | 91.0 | 86.0 | 81.0 | 103.0 |
| 1972 | 82.0 | 80.0 | 84.0 | 90.0 | 93.0 | 97.0 | 97.0 | 98.0 | 99.0 | 91.0 | 89.0 | 74.0 | 99.0 |
| 1973 | 77.0 | 75.0 | 86.0 | 84.0 | 93.0 | 93.0 | 97.0 | 94.0 | 91.0 | 91.0 | 85.0 | 78.0 | 97.0 |
| 1974 | 77.0 | 80.0 | 88.0 | 89.0 | 90.0 | 94.0 | 99.0 | 97.0 | 91.0 | 85.0 | 82.0 | 75.0 | 99.0 |
| 1975 | 81.0 | 79.0 | 81.0 | 87.0 | 90.0 | 92.0 | 98.0 | 96.0 | 96.0 | 91.0 | 83.0 | 78.0 | 98.0 |
| 1976 | 77.0 | 80.0 | 83.0 | 84.0 | 87.0 | 92.0 | 93.0 | 96.0 | 94.0 | 86.0 | 77.0 | 70.0 | 96.0 |
| 1977 | 73.0 | 85.0 | 83.0 | 82.0 | 93.0 | 97.0 | 100.0 | 98.0 | 92.0 | 92.0 | 83.0 | 79.0 | 100.0 |
| 1978 | 78.0 | 72.0 | 81.0 | 86.0 | 92.0 | 96.0 | 103.0 | 100.0 | 93.0 | 84.0 | 77.0 | 103.0 | |
| 1979 | 68.0 | 74.0 | 85.0 | 94.0 | 95.0 | 94.0 | 93.0 | 92.0 | 80.0 | 75.0 | 95.0 | ||
| 1980 | 72.0 | 83.0 | 83.0 | 87.0 | 92.0 | 99.0 | 103.0 | 104.0 | 100.0 | 90.0 | 85.0 | 75.0 | 104.0 |
| 1981 | 74.0 | 81.0 | 85.0 | 89.0 | 89.0 | 93.0 | 99.0 | 101.0 | 91.0 | 93.0 | 81.0 | 78.0 | 101.0 |
| 1982 | 77.0 | 84.0 | 87.0 | 85.0 | 92.0 | 93.0 | 97.0 | 100.0 | 99.0 | 90.0 | 84.0 | 80.0 | 100.0 |
| 1983 | 71.0 | 75.0 | 82.0 | 84.0 | 92.0 | 91.0 | 96.0 | 97.0 | 97.0 | 86.0 | 83.0 | 77.0 | 97.0 |
| 1984 | 74.0 | 75.0 | 87.0 | 87.0 | 93.0 | 98.0 | 99.0 | 104.0 | 99.0 | 86.0 | 85.0 | 78.0 | 104.0 |
| 1985 | 68.0 | 79.0 | 83.0 | 87.0 | 93.0 | 97.0 | 98.0 | 101.0 | 104.0 | 90.0 | 81.0 | 74.0 | 104.0 |
| 1986 | 77.0 | 90.0 | 82.0 | 85.0 | 90.0 | 95.0 | 101.0 | 103.0 | 92.0 | 91.0 | 83.0 | 69.0 | 103.0 |
| 1987 | 75.0 | 75.0 | 82.0 | 93.0 | 91.0 | 93.0 | 98.0 | 101.0 | 95.0 | 87.0 | 83.0 | 76.0 | 101.0 |
| 1988 | 72.0 | 79.0 | 82.0 | 88.0 | 92.0 | 99.0 | 99.0 | 100.0 | 95.0 | 86.0 | 83.0 | 75.0 | 100.0 |
| 1989 | 78.0 | 79.0 | 86.0 | 87.0 | 93.0 | 93.0 | 96.0 | 98.0 | 101.0 | 94.0 | 88.0 | 79.0 | 101.0 |
| 1990 | 77.0 | 80.0 | 82.0 | 92.0 | 92.0 | 99.0 | 101.0 | 104.0 | 102.0 | 94.0 | 82.0 | 78.0 | 104.0 |